a) Field of the Invention
The present invention relates to a sheet of plastic material which feels like and has at least some of the properties of paper, such as oxygen transmission rate, water vapor transmission rate and coefficient of friction, a process of producing same, as well as a composition for use in producing said sheet material. More particularly, the invention is concerned with a product which somewhat behaves and feels like paper, advantageously replaces food wrapping paper sheet material, can be used to replace stitched multi-wall paper bags, and finally is less expensive than paper.
b) Description of Prior Art
It is well known that commercial sheet material is a product which has found a very large variety of uses. Presently, the most popular products of this kind are either made of paper or plastic, although some uses have been found for sheets made of metal, such as aluminum. Of course, paper sheets and plastic sheets have different properties, plastic sheets being normally more resistant and more slippery than paper, while paper is normally much less water and air impervious. For purposes of handling and printing thereon, paper sheets are normally much more attractive than plastic sheets.
It will therefore be realized that there is a need for a sheet material that combines some of the properties of both paper sheets and plastic sheets.
A product of this nature has found its way on the market under the trademark Papermatch(copyright), and which is manufactured by A. Schulman Inc. The manufacturer claims that this plastic resin can be used for manufacturing packaging applications, and for labels, envelopes, folders and a variety of other uses. However, except for the fact that it may contain some granular mineral compounded with a resin, to applicant""s knowledge, there is no disclosure of a process of producing this material nor of its exact composition. There is therefore a need for a sheet material that feels like and has at least some of the specific properties of paper as well as for a process for producing the same.
It is therefore an object of the present invention to provide a sheet of plastic material which feels like and has at least some of the properties of paper. The sheet of plastic material according to the invention comprises a multilayer sheet having an outer layer, a middle layer, and an inner layer. The outer layer comprises between about 15 to 35 weight percent, the middle layer comprises between about 30 and 70 weight percent, and the inner layer comprises between about 15 to 35 weight percent, all with respect to the entire weight of the multilayer sheet. In addition, the outer layer comprises between about 40 to 90 weight percent polyethylene, between about 5 and 50 weight percent of a finely granulated material adapted to give a feel of paper to the multilayer sheet, and between about 5 and 10 weight percent of a pigment. The middle layer comprises between about 60 and 95 weight percent polyethylene, between about 0 and 20 weight percent of the finely granulated material, and between about 5 and 20 weight percent of pigment. Finally, the inner layer comprises between about 70 and 98 weight percent polyethylene, between about 0 and 10 weight percent of filler, and between about 0 and 20 weight percent of pigment.
Depending on the required properties of the multilayer sheet according to the invention, the different layers may comprise linear low polyethylene, high density polyethylene, or a mixture thereof. On the other hand, the choice of the finely granulated material is left to one skilled in the art, the preferred product for its convenience being calcium carbonate. A product which is of interest is a particulate calcium carbonate dispersed in a high density polyethylene base. Finally, although the choice of pigment is left entirely to one skilled in the art, the preferred material is titanium dioxide in powder form, although it may also include a beige pigment.
Some layers of the multilayer sheet according to the invention may include a filler as mentioned above. A preferred filler combination includes an anti-blocking agent which is chosen depending on the conditions prevailing in the extruder, silica, talc, diatomaceous earth, and any mixtures thereof.
The outer layer preferably comprises a linear low polyethylene, the middle layer preferably comprises a high density polyethylene, and the inner layer preferably comprises a linear low polyethylene.
For example, the outer layer may comprise between about 40 and 60 weight percent linear low polyethylene, between about 30 and 50 weight percent particulate calcium carbonate dispersed in a high density polyethylene base, and between about 4 and 8 weight percent of a pigment comprising titanium dioxide.
On the other hand, the middle layer may comprise high density polyethylene, preferably in an amount between about 75 and 85 weight percent, between about 10 and 20 weight percent of a particulate calcium carbonate dispersed in a high density polyethylene base, and titanium dioxide powder, preferably in an amount between 8 and 12 weight percent. Finally, the inner layer may comprise linear low polyethylene, for example between about 90 and 98 weight percent, and a filler comprising an anti-blocking agent, talc, silica, and diatomaceous earth, preferably in an amount between about 2 and 10 weight percent.
According to an alternate embodiment, the outer layer is made of high density polyethylene, preferably between about 80 and 90 weight percent, particulate calcium carbonate dispersed in a high density polyethylene, preferably between about 5 and 10 weight percent, and titanium dioxide powder, preferably between about 5 and 10 weight percent.
The middle layer, on the other hand, may comprise between about 80 and 90 weight percent low linear polyethylene, and between about 10 and 20 weight percent titanium dioxide pigment.
Finally, the inner layer, in this case may comprise between about 60 and 80 weight percent high density polyethylene, between about 10 and 20 weight percent low linear polyethylene, and between about 10 and 20 weight percent titanium dioxide pigment.
A preferred sheet material according to the invention has the following physical properties: an oxygen transmission rate between about 30 and 50 cc/100 in 2/24 hours, a water vapor transmission rate between about 0.01 and 0.025 and a coefficient of friction between about 0.4 and 0.5.
The invention also relates to a process of preparing a sheet multilayer sheet of plastic material which feels like and has at least some of the properties of paper, which comprises providing a first composition comprising between about 40 and 90 weight percent polyethylene, between about 5 and 50 weight percent of a finely granulated material adapted to give a feel of paper to the multilayer sheet, and between about 5 and 10 weight percent of a pigment, providing a second composition comprising between about 60 and 95 weight percent polyethylene between about 0 to 20 weight percent of the finely granulated material, and between about 5 and 20 weight percent of the pigment, and providing a third composition comprising between about 80 and 100 weight percent polyethylene, between about 0 and 10 weight percent of a filler, and between about 0 and 20 weight percent of the pigment, separately coextruding the first, second and third compositions and feeding the coextruded compositions to a die, and laminating same to constitute said multilayer sheet of plastic material.